I think there is currently no way to do this within PitStop. If your PDF creator application doesn't have the option, then you can use the Enfocus Browser application. You can find it on the Enfocus website under Support - Old product installers. It's an old application that hasn't been updated in a while but still works.

First you will need to enable editing in the preferences. Then go to the relevant page (there's a button for that). While holding the ctrl button (cmd on Mac) double-click one of the entries in the page dict. In the window that pops up select "Number" and change the "Dict Key" to "UserUnit" (no quotes, no spaces, exact casing), and the "Number value" to 2. Then click Put, and save the file (it's probably best to save a copy in case something goes wrong).